Generally, when an induction electric motor is driven by a convention inverter apparatus, a variable voltage--variable frequency control (hereinafter is referred to as VVVF control) is used. The VVVF control controls a magnetic flux of the motor so as to be always constant even when the output frequency of the inverter apparatus is varied. The VVVF control also controls fundamentally a ratio of a voltage applied to the motor and a frequency applied to the motor such that the ratio remains constant. Thus, the ratio V/F of an output voltage V and an output frequency F of the inverter apparatus is made constant.
An example of the conventional inverter apparatus is shown in FIG. 1. Numeral 1 designates a three phase or single phase power source, in this case a three phase power source. Numeral 2 designates a converter part (regular conversion part) of the inverter apparatus, and numeral 3 designates an inverter part (reverse conversion part). Numeral 4 designates the induction electric motor, which is three-phase coupled with inverter part 3. Numeral 12 is a PWM signal generating circuit which generates a pulse width modulating signal (hereinafter is referred to as PWM signal) of width proportional to an output frequency signal from an outside source. The PWM signal generated by PWM signal generating circuit 12 is transmitted to driver circuit 10 through a photo-coupler or a relay, and is input either as a base input signal or a gate input signal of a switching element such as power transistor, thyristor of inverter part 3 through driver circuit 10.
PWM signal generating circuit 12 has a memory and a microcomputer. An example of the relation of the PWM signal and the three-phase terminal voltage is shown in FIG. 2a and 2b. FIG. 2a shows a signal of inverter part 3 in a high output frequency, and FIG. 2b shows a signal of inverter part 3 in a low output frequency. The horizontal direction in FIG. 2a and FIG. 2b shows time and a vertical direction shows output values. In FIG. 2a, numeral 13 designates a triangular wave signal, and numerals 14, 15, 16 designate sinusoidal wave signals. The sinusoidal wave signals 14, 15, 16 have a phase difference of 120.degree. with respect to each other. A U-phase terminal voltage Vu is obtained by voltage comparison of sinusoidal wave signal 14 and triangular wave 13. A V-phase terminal voltage Vv is obtained by voltage comparison of sinusoidal wave signal 15, having a phase difference of 120.degree., to the sinusoidal wave signal 14. Finally, a W-phase terminal voltage Vw is obtained by voltage comparison of a sinusoidal wave signal 16, having phase difference of 120.degree., to the sinusoidal wave signal 15. When VVVF control is made, a ratio of wave height values of the sinusoidal wave signals 14, 15, 16 and a wave height value of the triangular wave signal 13 is changed by means of frequency. As a result, in high frequency (a state of FIG. 2a) the voltage applied to motor 4 is increased, and in low frequency (a state of FIG. 2b ) the voltage is reduced, such that the controlled ratio of the motor applying voltage and the frequency is usually constant.
By the above-mentioned principle, the PWM signal is generated by PWM signal generating circuit 12, and a switching timing of inverter part 3 is determined. In order to prevent a short circuit phenomenon by simultaneous trigger of the upper and lower elements which are coupled in series to the plural switching elements of inverter part 3, a short circuit preventing period td is set. The short circuit preventing period td is described in FIG. 6 of page 28 (834) of "MITSUBISHI DENKI GIHO" Vol. 58.No. 12.1984, and therefore the details are omitted.
A problem with the short circuit preventing period td is that it generates an error voltage corresponding to an output current from driver circuit 10 and distorts the output voltage waveform of inverter part 3. As the output voltage is lowered, the distortion increases; further, as the switching frequency (hereinafter is referred to as a carrier frequency) of the upper and lower elements increases, wherein the upper and lower elements are connected in series with the switching elements of the inverter part 3, the distortion increases. Thus, there is a problem of generating a pulsation of torque in induction electric motor 4. Though it is possible to lower the carrier frequency to overcome the pulsating torque, in the case that the carrier frequency is lowered, there is a possibility that the pulsation or vibration of the induction electric motor enters the audio frequency range, thus generating noise.
In order to solve the above-mentioned problem an inverter apparatus having a constitution as shown in FIG. 5 of page 27 (833) of "MITSUBISHI DENKI GIHO" Vol. 58.No. 12.1984 is considered. The constitution is that a voltage applied to the electric motor is detected through an instant voltage detecting circuit; a signal which is output from a sinusoidal wave reference voltage circuit on the basis of a V/F command signal is compared with a signal which is output from the instant voltage detecting circuit. The two signals are then input to the PWM circuit to drive the inverter part on the basis of the inputted compared signal. Since only the distortion of the output waveform can be corrected by feedback of the applied voltage, the carrier frequency can be raised up to 20 KHz, so that a noiseless state is realized. However, since the VVVF control of constant ratio V/F is being used, in the case where a power source voltage supplied to the inverter part is varied, a different output voltage is generated to the same output frequency, thus causing instability.